Copper beryllium alloys have now been in commercial use for approximately fifty years and have a well-deserved reputation for being useful in many applications requiring high strength, formability, stress relaxation resistance and conductivity. A number of commercial copper beryllium alloys are known, including those bearing Copper Development Association designations C17500, C17510, C17000, C17200, C17300 as well as C81300, C82100, C82200 and C82400. These alloys contain varying amounts of beryllium and other alloying elements such as cobalt, nickel, silver, etc. In general, development of beryllium copper alloys has proceeded in the direction of providing premium performance, i.e., the highest strengths, best ductilities and the other highly desirable attributes of those alloys which are age-hardenable. Thus, U.S. Pat. Nos. 1,893,984, 1,957,214, 1,959,154, 1,974,839, 2,131,475, 2,166,794, 2,167,684, 2,172,639, 2,289,593 and 3,376,171 disclose various alloys containing varying amounts of beryllium and other elements. Various applications for such alloys are also discussed in the art. For example, spot welding electrodes are described in U.S. Pat. Nos. 1,957,214, 1,959,154 and 2,131,475. The latter patent sought to provide a satisfactory electrode with reduced amounts of cobalt and beryllium.
In the fifty or so years since the above-discussed patents were granted, whole new industries have appeared and new sets of requirements have been imposed on alloy producers. Thus, the requirements of the electronics and computer industries were unknown in the 1930's. Even the trends toward miniaturization in electronics and computers have arisen and proceeded at an accelerating pace only in the past few years. In the provision of spring-type connectors and contacts, the complexity of the devices needed, and the requirements for heat dissipation, as well as for survival of parts at elevated temperatures without failure due to stress relaxation, have proceeded apace. In addition, purchasers have become increasingly price-conscious and connector alloys such as the phosphor bronzes have been employed due to cost even though the inferior performance of such alloys, such as in poorer conductivity, poorer formability and lower stress relaxation resistance as compared to beryllium copper alloys, was known. Moreover, the formability requirements, which are of importance in the production of complex parts from strip or wire using progressive dies or other forming technologies, have elevated the difficulties imposed upon alloy suppliers as compared to the simpler days of U.S. Pat. No. 2,131,475 wherein the welding electrode described is merely a bar of wrought or cast metal which had to resist "mushrooming" under load, but with respect to which no formability requirement was imposed.
The property of stress relaxation is an important design parameter which can give the designer assurance that a particular contact or connector or like device will maintain the required contact pressure at elevated temperatures to assure long-life performance of an assembly including the device. Stress relaxation is defined as the decrease of stress at constant strain with time for a given temperature. From a knowledge of the stress relaxation behavior of a material, a designer can determine how much the room temperature spring force must be increased to assure a particular minimum force at operating temperature to maintain electrical contact between mating parts for an extended time period.
The stronger beryllium-containing age hardenable alloys such as C17200, which contains about 2% beryllium, are known to have high resistance to stress relaxation. On the other hand, the considerably cheaper phosphor bronzes, such as C51000 and C52100, which are not age hardenable and have to be severely cold worked to achieve high strength, are poor with respect to resistance to stress relaxation.
As used herein, stress relaxation resistance is determined by the test described in the paper entitled "Stress Relaxation of Beryllium Copper Strip In Bending" by Harkness and Lorenz presented at the 30th Annual Relay Conference, Stillwater, Oklahoma, Apr. 27-28, 1982. In accordance with this test, flat spring specimens having a contoured gage length are loaded in a fixture to a constant initial stress level and are exposed with the fixture in the stressed condition to an elevated temperature such as 300.degree. F. (150.degree. C.) for an extended time period. Periodically, a specimen is removed and measured to determine the amount of permanent set the material has undergone, from which the percent of remaining stress value can be calculated.
Formability is determined by bending a flat strip specimen, for example by 90.degree., about a punch having a nose of variable known radius with failure being taken as the point at which cracking occurs in the outer fibers of the bend. A rating is given for the test from the quantity R/t wherein "R" is the radius of the punch nose and "t" is the thickness of the strip. The rating can be used by designers to determine whether a particular material can be formed to the geometry desired in a particular part.
Technical papers dealing with copper beryllium alloys include: Rioja and Laughlin in Acta Metallurgica, Vol. 28, 1980; Laughlin and Tanner in Bulletin of Alloy Phase Diagrams, Vol. 2, No. 1, 1981; Guha, Alexander and Laughlin in "Metastable Precipitation in Ternary Cu--Ni--Be and Cu--Co--Be Alloys", presented at the TMS-AIME Fall Meeting, St. Louis, Mo., Oct. 25, 1982; and Chang, Neumann, Mikula and Goldberg in INCRA Monograph Series VI, 1979.
The invention provides an age hardenable copper beryllium alloy having a stress relaxation resistance closely approaching that of the strongest copper beryllium alloys of commerce together with high formability and ductility, high conductivity and useful strength, together with a previously unknown metallurgical structure.